In gravity aided liquid dispensing systems, such as for dispensing dairy products, the pressure (referred to as head pressure) of the liquid over the dispensing valve at the bottom of the tank changes with the liquid column height. Accordingly, the rate of flow of liquid through the dispensing changes as the head pressure decreases. Those skilled in the art should understand there are well known mathematical calculations which can be made to determine how much time is required to dispense via gravity, an approximate volume of a liquid having known properties from a tank of known dimensions through an outlet of known dimensions, when a weight or height level of the liquid in the tank is known. Therefore having knowledge of the liquid level allows for adjustment of the opening time of the valve to obtain a substantially constant volume of dispensed dairy product.
In certain applications, such as dairy dispensing for coffee consumption, it is necessary to dispense predetermined volumes, or shots, of dairy product for consistent user taste experience, where different predetermined volumes of dairy product can be selected for dispensing. Hence, accuracy in determining the liquid level in the tank is critical for ensuring consistent volumes of the dairy product are dispensed as the tank drains. It is well-known that the flow rate of liquid through an outlet via gravity changes as the head pressure changes due to the drop in liquid in the tank.
Currently known solutions for determining the level of liquid in a tank include the use of one or more load cells or pressure sensors to measure the weight of the tank and its content to assess the pressure caused by the liquid on the dispensing valve at the front of the dispenser, as disclosed in U.S. Pat. No. 8,534,497. There are several disadvantages to using load cells in such an application when it comes to weighing the tank and its content. For example, the calibration of the load cells may need to take into account temperature, as the liquid to be dispensed can be refrigerated, warm, or left at ambient temperature. Accumulation of residual product on the walls of the tank will adversely affect the measurements, and correction is required if measurements are performed at the back of the dispenser and if the tank bottom is slightly inclined toward the front of the dispenser, and/or if the dispenser itself is not at level.
Other issues can include variability of tank weight when they are changed, which can affect the calibration and may require a zero point setting operation. Converting from mass to liquid column height can induce errors as well, based on potential inconsistent geometry of the liquid container caused by production variances, aging, or future design changes. Converting from mass to liquid column height also introduces potential errors and the complexity of having to consider the specific gravity of the liquid.
Hence, optical based liquid level detection systems have been proposed. Some known optical liquid level detection systems require immersion of the detector itself into the liquid of the tank, which is highly undesirable and sometimes not permitted in applications where the stored liquid is to be consumed as contamination of the liquid can occur if the detector is not properly cleaned. The use of photodiodes affixed to the dairy container to sense ambient light or light from light sources such as LED and laser diode have also been proposed. There are multiple drawbacks to such known optical liquid level detection systems.
FIG. 1 shows an optical liquid height determining system of the prior art. In FIG. 1, a tank 10 is shown from a front end view, and is generally rectangular in shape to hold liquid inside. The tank 10 has a top 12 which can be covered to prevent contaminants from entering the stored liquid. Extending from a bottom of tank 10 is an outlet nozzle 14 that allows liquid to flow out via gravity. While not shown, outlet nozzle 14 can be connected to a valve that controls the flow of liquid. Affixed to the side wall 16 is an array of light sensors such as photodiodes 18, of which only one is numbered. The photodiodes 18 are generally arrayed in a vertical direction along the side wall 16 such that each photodiode 18 is positioned at a different known height of the tank. Affixed to the opposite side wall 20 is an array of light sources 22, of which only one is numbered. There can be any number of light sources 22 attached to the side wall 20, provided each of the photodiodes 18 can receive a sufficient amount of light when the liquid drops below its particular level. In operation, the photodiodes 18 and the light sources 22 are turned on to determine the height of the liquid surface 24.
The principle of operation is as follows. Any photodiode 18 above the liquid surface 24 will detect an amount of light, and thereby generate a corresponding electrical signal indicating the absence of liquid at that particular photodiode 18. Any photodiode 18 below the liquid surface 24 will receive an amount of light below said threshold, and thereby generate a corresponding electrical signal that will be less than a photodiode 18 above the liquid surface 24. These signals can then be analyzed by a preprogrammed controller or microcontroller. Therefore, actual height of the liquid surface 24 in the tank 10 can be estimated by identifying the first photodiode 18 from the bottom of the tank 10 generating an electrical signal greater than the other photodiodes 18 below the liquid surface 24. Alternately, each photodiode 18 can simply generate an electrical signal when the amount of light it detects exceeds a specific threshold, and does not generate any signal in the amount of light it detects is below said specific threshold.
Accordingly, this can be seen as a “digital” liquid height determination system as multiple detectors (over two dozen) are needed to reach accuracy below 1 cm over a 30 cm liquid level range, as each detector simply determines the presence or absence of sufficient light at its height level. Therefore the resolution of the liquid height determination is limited to the spacing between detectors, which increases the component costs and data acquisition complexity. Furthermore in such systems, the tank needs to be modified to fix the detectors and light sources to its walls. This needs to be performed for every tank to be used in a dispenser including replacement tanks that are used when performing cleaning of the tank. This increases the cost of each tank and necessitates connection and disconnection of wires to communicate data signals and power. Hence this introduces the possibility of failures due to damaged connectors or improperly attached connectors. Additionally, due to the direct attachment of the detectors and light sources to the tank walls, such threshold based light detection leads to the problem where residues forming on the wall surface can provide a false positive output by blocking sufficient light from being detected by the adjacent photodiode, thereby “fooling” it into providing an incorrect output when the actual liquid level has dropped to a lower level.
Furthermore, light sources affixed to the walls of the tank will increase the wall temperature from heat dissipation, and thereby increase the probability of bacterial growth and possible contamination of dairy product stored in the tank. In the case where non-dairy products are stored in the tank, refrigeration of the product may still be desired. Unfortunately, the temperature dependent response of photodiode emitters and detectors could be impacted by the liquid level that will be at a different temperature than ambient temperature, thereby inducing a difference between detectors by up to 10° C. As any person skilled in the art would appreciate, such variation introduces calibration complexity which if not done properly, will result in an accurate dispenses of the liquid.
It is, therefore, desirable to provide a liquid level determination system that provides consistent and accurate measurements for any type of liquid and at any temperature without the problems of prior systems as noted above, and at low cost.